340,000 stars’ DNA interrogated in search for Sun’s lost siblings

ASTRO 3D researchers have revealed the “DNA” of more than 340,000 stars in the Milky Way, which should help them find the siblings of the Sun, now scattered across the sky.

This is a major announcement from an ambitious Galactic Archaeology survey, called GALAH, launched in late 2013 as part of a quest to uncover the formulation and evolution of galaxies. When complete, GALAH will investigate more than a million stars.

The GALAH survey used the HERMES (High Efficiency and Resolution Multi-Element Spectrograph) instrument at the Australian Astronomical Observatory’s (AAO) 3.9-metre Anglo-Australian Telescope near Coonabarabran, NSW, to collect spectra for the 340,000 stars.

The GALAH survey is the brainchild of Professor Joss Bland-Hawthorn from the University of Sydney and the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and Professor Ken Freeman of the Australian National University (ANU). It was conceived more than a decade ago as a way to unravel the history of our Milky Way galaxy; the HERMES instrument was designed and built by the AAO specifically for the GALAH survey.

The ‘DNA’ collected traces the ancestry of stars, showing astronomers how the Universe went from having only hydrogen and helium – just after the Big Bang – to being filled today with all the elements we have here on Earth that are necessary for life.

GALAH abundance exploration – the panels show the abundances of 20 different elements measured from GALAH data for 7,626 stars for which distances, ages and Galactic orbits are known quite precisely. This data allows us to study the production of elements over time in the Milky Way. the top right panel showing so-called “alpha elements” versus iron abundance, is a key indicator of element production in supernova explosions. The colour scale represents the density of stars in each panel and black dots show abundance for dwarf stars from previous higher-resolution studies with samples of fewer than 1,000 stars. IMAGE: Sarah Martell

Dr. Sarah Martell from the UNSW Sydney, who leads GALAH survey observations, explained that the Sun, like all stars, was born in a group or cluster of thousands of stars.

“Every star in that cluster will have the same chemical composition, or DNA – these clusters are quickly pulled apart by our Milky Way Galaxy and are now scattered across the sky,” Dr. Martell said.

“The GALAH team’s aim is to make DNA matches between stars to find their long-lost sisters and brothers.”

For each star, this DNA is the amount they contain each of nearly two dozen chemical elements such as oxygen, aluminium, and iron.

Unfortunately, astronomers cannot collect the DNA of a star with a mouth swab but instead use the starlight, with a technique called spectroscopy.

The light from the star is collected by the telescope and then passed through an instrument called a spectrograph, which splits the light into detailed rainbows, or spectra.

Professor Martin Asplund from ANU and a Chief Investigator with ASTRO 3D, said GALAH, when complete, would help reveal the original star clusters of the Milky Way, including the Sun’s birth cluster and solar siblings along with more than one million other stars.

“This survey allows us to trace the ancestry of stars, showing astronomers how the Universe went from having only hydrogen and helium – just after the Big Bang – to being filled with all the elements we have here on Earth that are necessary for life,” said Professor Asplund.

Every star in the Sun’s birth cluster will have the same chemical composition – this cluster was quickly pulled apart by our Milky Way Galaxy and scattered across the sky.

Professor Asplund led the analysis of the survey data with PhD students Mr Sven Buder from the Max Planck Institute for Astronomy in Germany and Ms Ly Duong from ANU.

“Measuring each chemical element abundance to get the stellar DNA for so many stars is an enormous challenge, but that’s exactly what we have done so this is a fantastic scientific achievement,” Professor Asplund said.

The GALAH team have spent more than 280 nights at the telescope since 2014 to collect all the data.

Map of GALAH survey progress through to October 2017. Clear circles are unobserved survey fields, pink are regular survey fields that have been observed, cyan are field observed during the Pilot Survey, blue are fields observed by the K2‑HERMES programme, and purple are fields observed for the targeted Tycho‑2 bright‑star subproject. IMAGE: Sarah Martell